A crossing predictor (often referred to as a grade crossing predictor in the U.S. or a level crossing predictor in the U.K.) is an electronic device that is connected to the rails of a railroad track and is configured to detect the presence of an approaching train and determine its speed and distance from a crossing (i.e., a location at which the tracks cross a road, sidewalk or other surface used by moving objects), and use this information to generate a constant warning time signal for controlling a crossing warning device. A crossing warning device is a device that warns of the approach of a train at a crossing, examples of which include crossing gate arms (e.g., the familiar black and white striped wooden arms often found at highway grade crossings to warn motorists of an approaching train), crossing lights (such as the red flashing lights often found at highway grade crossings in conjunction with the crossing gate arms discussed above), and/or crossing hells or other audio alarm devices. Crossing predictors are often (but not always) configured to activate the crossing warning device at a fixed time (e.g., 30 seconds) prior to an approaching train arriving at a crossing.
Typical crossing predictors include a transmitter that transmits a signal over a circuit formed by the track's rails and one or more termination shunts positioned at desired approach distances from the transmitter, a receiver that detects one or more resulting signal characteristics, and a logic circuit such as a microprocessor or hardwired logic that detects the presence of a train and determines its speed and distance from the crossing. The approach distance depends on the maximum allowable speed of a train, the desired warning time, and a safety factor. Preferred embodiments of crossing predictors generate and transmit a constant current AC signal on said track circuit; the crossing predictor detects a train and determines its distance and speed by measuring impedance changes caused by the train's wheels and axles acting as a shunt across the rails, which effectively shortens the length (and hence the impedance) of the rails in the circuit.
To prevent the signals transmitted by one crossing predictor from interfering with another crossing predictor, crossing predictors are often configured to transmit on different frequencies. Hence, crossing predictors use frequency specific termination shunts to define their approach length. These termination shunts are set to a fixed frequency (i.e., the termination frequency) to match the frequency of the crossing predictor, but the shunts may be equipped with switches and/or jumpers to allow their termination frequency to be manually changed in the field if need be. These shunts, however, are typically buried or located in wayside enclosures some distance (e.g., 3,000 feet) away from the crossing predictor equipment. Thus, changing the termination frequency of the shunts can be difficult and time consuming, thereby slowing rail and vehicle traffic during the changing process, and would require an undesirable amount of man-power to complete. There is, therefore, a need and desire for a better mechanism for changing the termination frequency of an installed termination shunt.